Flow measurement and control with bubble detection

ABSTRACT

Systems and methods for liquid flow sensing and control for use with a variety of different types of liquid flow measurement and control systems. The liquid flow sensor system senses a flow signal indicative of the flow rate of the liquid flowing in a sensor conduit and analyzes the flow signal to determine, by detecting characteristic changes in the signal, whether a bubble is present in the sensor conduit. Where the system determines that a bubble is present, it may generate an alarm signal indicative of the presence of the bubble. A flow control system incorporating the flow sensor as a feedback source may respond to the detection of a bubble by temporarily freezing the flow control parameters until the bubble has exited the sensor conduit. The flow control system can implement procedures for clearing a bubble from the sensor conduit where the system detects that the bubble has become stuck.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 60/707,628, filed Aug. 12, 2005,entitled “Bubble Algorithm Design,” which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to systems and methods for measuring andcontrolling liquid flow, and more specifically to systems and methodsfor measuring and controlling liquid flow in environments where bubblesmay be present in the liquid flow.

2. Discussion of Related Art

Several techniques exist for measuring a flow rate of a liquid flowingin a conduit, pipe, or tube. These include thermal flow meters, coriolisforce flow meters, differential pressure flow meters, and ultrasonicflow meters. Generally, liquid flow meters sense one or more parametersof the flow that can be calibrated to correspond to the rate (e.g.,volumetric or mass) of flow of the liquid. Such flow meters can be usedeither as passive monitors of liquid flow through a system of interest(i.e., used as a flow meter), or as a sensor element in a closed-loopfeedback system that controls liquid flow in the system of interest(i.e., as a flow meter in a flow rate controller).

SUMMARY OF INVENTION

When a bubble enters a sensor conduit of any type of flow meter, thesignal upon which the flow meter bases its flow measurement may bedisturbed. This can result in spurious measurements. Where the flowmeter is operating as the sensor element of a closed-loop flowcontroller, the closed loop system may react to the spurious signal andattempt to compensate for what it interprets as a change in the flowrate of the liquid. This can lead to instability in the controlled flow,which interferes with the purpose of the controller to provide a stableflow at a rate corresponding to a (typically user-set) setpoint.

Although the presence of bubbles can affect any type of flow meter orcontroller (and to varying degrees), it is especially problematic forultrasonic-type flow meters, particularly where such ultrasonic flowmeters are used to measure or control relatively low flow rates (e.g.,between 5 to 50 ml/min, or less). Applicants have recognized thatconventional ultrasonic flow meters and flow controllers lack theability to detect and respond to the presence of a bubble in a sensorconduit and to remain in a stable state despite the presence of thebubble in the sensor conduit, especially where they are used to measureand/or control such relatively low flow rates.

Applicants have developed systems and methods for achieving stableoperation of liquid flow controls even when a bubble passes through thesensor system. The systems and methods developed by applicants canminimize the effect of the bubble on liquid flow measurements or oncontrolled liquid flow rates.

In exemplary embodiments, a liquid flow sensor system of the presentinvention senses a flow signal indicative of the flow rate of the liquidflowing in a sensor conduit. The system analyzes the flow signal todetermine, for example, by detecting characteristic changes in thesignal, whether a bubble is present in the sensor conduit. Where thesystem determines that a bubble is present, it may generate an alarmsignal indicative of the presence of the bubble. In exemplaryembodiments of a flow control system that incorporates the flow sensoras a feedback source for closed-loop control and includes bubbledetection, the flow control system may respond to the detection of abubble by temporarily freezing the flow control parameters until thebubble has exited the sensor conduit. In this way, the system may avoidinstabilities in flow rate caused by the closed-loop flow controlattempting to track spurious flow sensor signals caused by the presenceof the bubble.

In accordance with an aspect of the present invention, a method ofmeasuring a flow rate of a liquid flowing in a liquid flow sensor isprovided, the liquid flow sensor including a sensor conduit and theliquid including a plurality of bubbles formed therein. The methodcomprises acts of repeatedly sensing a flow signal indicative of theflow rate of the liquid flowing in the sensor conduit; determining,based upon at least one parameter of the flow signal, whether at leastone bubble is disposed in the liquid within the sensor conduit;providing, in response to a determination that no bubble is disposed inthe liquid within the sensor conduit, a flow rate signal indicative ofthe flow rate of the liquid flowing in the liquid flow sensor based upona most recently sensed flow signal; and providing, in response to adetermination that the at least one bubble is disposed in the liquidwithin the sensor conduit, at least one of: a) a flow rate signal basedupon the most recently sensed flow signal and an alert signal indicativeof a presence of the at least one bubble, and b) a flow rate signalindicative of the flow rate of the liquid flowing in the liquid flowsensor based upon other than the most recently sensed flow signal.

In accordance with one embodiment wherein the liquid flow sensorcomprises an ultrasonic liquid flow sensor, the at least one parameterof the flow signal comprises an ultrasonic flow signal amplitude. Inaccordance with this embodiment, the method may further comprise an actof calculating, responsive to the act of repeatedly sensing, a weightedsum, such as a running average, of a value of the ultrasonic flow signalamplitude, wherein the act of determining includes an act of determiningwhether a most recently sensed value of the ultrasonic flow signalamplitude deviates from the weighted sum by more than a determinedamount.

In accordance with another embodiment wherein the liquid flow sensorcomprises an ultrasonic liquid flow sensor, and wherein the flow signalcomprises an ultrasonic time-of-flight difference flow signal, the atleast one parameter of the flow signal includes a magnitude of theultrasonic time-of-flight difference flow signal. In accordance with afurther embodiment, the act of determining includes an act ofdetermining whether the magnitude of the ultrasonic time-of-flightdifference flow signal deviates from the magnitude of a prior ultrasonictime-of-flight difference flow signal by more than a threshold value. Inaccordance with a still further embodiment of the present invention, themagnitude of the ultrasonic time-of-flight difference flow signal may beused in combination with detection of a bubble based upon the ultrasonicflow signal amplitude to provide early detection of the presence of abubble in the sensor conduit.

In accordance with another aspect of the present invention, a method ofcontrolling the flow rate of a liquid through a flow conduit that iscoupled to a controllable valve and a liquid flow sensor is provided.The liquid flow sensor includes a sensor conduit and the liquid includesa plurality of bubbles formed therein, and the method comprises acts ofrepeatedly sensing a flow signal indicative of the flow rate of theliquid flowing in the sensor conduit; determining, based upon at leastone parameter of the flow signal, whether at least one bubble isdisposed in the liquid within the sensor conduit; providing, in responseto a determination that no bubble is disposed in the liquid within thesensor conduit, control parameters for the controllable valve based uponthe most recently sensed flow signal; providing, in response to adetermination that the at least one bubble is disposed in the liquidwithin the sensor conduit, control parameters for the controllable valvebased upon other than the most recently sensed flow signal; andcontrolling the controllable valve according to the control parameters.

In accordance with one embodiment, the method further includes an act ofcalculating, responsive to the act of repeatedly sensing, a weightedsum, such as a running average, of a value of the at least one parameterof the flow signal, wherein the act of determining includes an act ofdetermining whether a most recently sensed value of the at least oneparameter of the flow signal deviates from the weighted sum by more thana determined amount. In accordance with another embodiment, the act ofproviding control parameters based upon other than the most recentlysensed flow signal includes providing the most recently provided controlparameters for which it was previously determined that no bubble wasdisposed in the liquid within the sensor conduit.

In accordance with yet a further embodiment, the method furthercomprises acts of waiting a predetermined period of time in response toa determination that the at least one bubble is disposed in the liquidwithin the sensor conduit, determining, after the predetermined periodof time, whether the at least one bubble is still disposed in the liquidwithin the sensor conduit, based upon the at least one parameter of theflow signal, and executing, in response to a determination that the atleast one bubble is still disposed in the liquid within the sensorconduit, a controlled force procedure to remove the at least one bubblefrom the liquid within the sensor conduit. In one embodiment, thecontrolled force procedure includes opening and shutting thecontrollable valve.

In accordance with a further aspect of the present invention, a systemfor measuring a flow rate of a liquid flowing in a flow conduit isprovided for a liquid that may include a plurality of bubbles formedtherein. The system comprises a liquid flow sensor that includes asensor conduit fluidly coupled to the flow conduit and a bubbledetection module. The liquid flow sensor is configured to sense a flowrate of the liquid flowing in the sensor conduit and provide a flowsignal indicative of the flow rate of the liquid flowing in the sensorconduit. The bubble detection module is coupled to the liquid flowsensor to receive the flow signal and determine, based upon at least oneparameter of the flow signal, whether at least one bubble is disposed inthe liquid within the sensor conduit. The bubble detection module isconfigured to provide, in response to a determination that no bubble isdisposed in the liquid within the sensor conduit, a flow rate signalindicative of the flow rate of the liquid flowing in the liquid flowsensor based upon a most recently sensed flow signal, and to provide, inresponse to a determination that the at least one bubble is disposed inthe liquid within the sensor conduit, at least one of: a) a flow ratesignal based upon the most recently sensed flow signal and an alertsignal indicative of a presence of the at least one bubble, and b) aflow rate signal indicative of the flow rate of the liquid flowing inthe liquid flow sensor based upon other than the most recently sensedflow signal.

In accordance with one embodiment directed to a flow controller, thesystem further comprises a controllable valve in fluid communicationwith the flow conduit, to control the flow rate of the fluid flowing inthe flow conduit based upon control parameters provided to thecontrollable valve, and a controller, coupled to the liquid flow sensorand the controllable valve, to receive the flow signal from the liquidflow sensor and provide the control parameters to the controllablevalve. In accordance with one embodiment, the bubble detection module isimplemented in the controller, wherein, upon the determination that nobubble is disposed in the liquid within the sensor conduit, thecontroller provides the control parameters to the controllable valvebased upon the most recently sensed flow signal, and wherein, upon adetermination that the at least one bubble is disposed in the liquidwithin the sensor conduit, the controller provides the controlparameters to the controllable valve based upon other than the mostrecently sensed flow signal.

In an alternative embodiment directed to a flow controller, wherein thebubble detection module provides the flow rate signal based upon themost recently sensed flow signal and the alert signal indicative of thepresence of the at least one bubble in response to the determinationthat the at least one bubble is disposed in the liquid within the sensorconduit, the system further comprises a controllable valve in fluidcommunication with the flow conduit, to control the flow rate of thefluid flowing in the flow conduit based upon control parameters providedto the controllable valve, and a controller, coupled to the bubbledetection module, to receive the flow rate signal and the alert signaland provide the control parameters to the controllable valve.

In a further embodiment, in response to the alert signal, the controllerfreezes the control parameters provided to the controllable valve at aprior value. In yet another embodiment, in response to the alert signal,the controller is configured to wait for a predetermined period of time,and upon a determination that the at least one bubble is still disposedin the liquid within the sensor conduit and the predetermined period oftime has elapsed, implement a controlled force procedure to remove theat least one bubble from the liquid within the sensor conduit.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments of systems and methods according to the presentinvention will be understood with reference to the accompanyingdrawings, which are not intended to be drawn to scale. In the drawings,each identical or nearly identical component that is illustrated invarious figures is represented by a like designator. For purposes ofclarity, not every component may be labeled in every drawing. In thedrawings:

FIG. 1 is a schematic representation of an exemplary embodiment of aliquid flow measurement and/or control system;

FIG. 2 is a flow diagram illustrating an exemplary embodiment of amethod of detecting and responding to the presence of a bubble in thesensor conduit of a liquid flow measurement and/or control system;

FIG. 3 is a flow diagram illustrating an exemplary embodiment of amethod of detecting and responding to the exit of a bubble from thesensor conduit of a liquid flow measurement and/or control system;

FIG. 4 is a flow diagram illustrating an exemplary embodiment of amethod of detecting the presence of a bubble in the sensor conduit of aliquid flow system;

FIG. 5 is a flow diagram illustrating an exemplary embodiment of amethod of detecting the presence of a bubble in the sensor conduit of aliquid flow system;

FIG. 6 is a flow diagram illustrating an exemplary embodiment of amethod of detecting the presence of a bubble in the sensor conduit of aliquid flow system;

FIG. 7A illustrates a signal in an ultrasonic sensor conduit before abubble enters the sensor conduit;

FIG. 7B illustrates a signal in an ultrasonic sensor conduit as a bubblereaches the entrance of the sensor conduit;

FIG. 7C illustrates a signal in an ultrasonic sensor conduit as a bubblepasses a first transducer;

FIG. 7D illustrates a signal in an ultrasonic sensor conduit as a bubbletraverses the middle of the conduit;

FIG. 7E illustrates a signal in an ultrasonic sensor conduit as a bubblereaches a second transducer;

FIG. 7F illustrates a signal in an ultrasonic sensor conduit as a bubblereaches the exit end of the conduit;

FIG. 7G illustrates a signal in an ultrasonic sensor conduit after abubble has exited the sensor conduit;

FIG. 8 illustrates an ultrasonic flow sensor signal plotted against timeas a bubble traverses the sensor conduit of the flow sensor;

FIG. 9 illustrates a differential time-of-flight signal obtained from anultrasonic flow sensor, plotted against time, as a bubble traverses thesensor conduit of the flow sensor; and

FIG. 10 illustrates an exemplary embodiment of an ultrasonic flow metersensor conduit that may be used in accordance with the presentinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, there is illustrated a schematic block-diagram ofone example of a liquid control system according to aspects of thepresent invention. The system 100 as illustrated includes a controllablevalve 108 through which liquid flows, as indicated by line/arrow 104. Itis to be appreciated that although the following discussion will referprimarily to element 108 being a controllable or variable valve, element108 may also be another type of liquid actuator such as, for example, apump. The valve 108 may be, for example, an electronically controlledvariable valve that may be adjusted to vary the flow rate of the liquidthrough the system. The valve 108 is controlled by a controller 120 asindicated by line 114. The controller 120 may be, for example, amicroprocessor-based controller. A liquid flow meter 110 may bepositioned downstream of the valve 108, as shown. Alternatively, theliquid flow meter 110 may be disposed upstream of the valve 108. Theflow of the liquid may be measured by the flow meter 110 that maycommunicate with the controller 120, either directly via line 119, orindirectly as indicated by lines 118 and 122. As illustrated in FIG. 1,in one example, the flow meter 110 may be integral to the liquid line104, such that liquid flowing in the liquid line 104 also flows throughthe flow meter 110. It is to be appreciated that the flow meter 110 maybe integral with the liquid line 104, or may be positioned in a branchor bypass liquid line such that the flow meter 110 may measure only aportion of the entire liquid stream.

In a conventional liquid flow control system, signals from the flowmeter indicative of the liquid flow rate may be sent to controller 120,and the controller 120 may be adapted to use information, such as theflow rate of the liquid, provided by the flow meter 110 to monitor theflow rate of the liquid and to control the valve 108 to achieve adesired rate of flow, thereby providing closed-loop control of theliquid flow in the system 100. For example, in one embodiment, controlof the liquid flow rate is achieved by a control system in which theliquid flow meter 110 is a feedback element in a closed loop system. Theflow meter 110 produces an electronic signal indicative of the rate offlow of the liquid through the liquid line 104. The flow meter signalmay provide real-time feedback of liquid flow and may be input to thecontroller 120. A signal provided by the controller 120 is input to anactuator driving the valve 108 (as indicated by line 114) and may beused to control the valve 108 so as to vary the rate of flow as requiredto achieve the desired rate of liquid flow. The desired liquid flow ratemay also be an input parameter to the controller 120. For example, theliquid may enter the illustrated portion of the system 100 from a source102 which may be, for example, an upstream element in the system 100, astorage element, and the like. The source 102 may also include sensorsthat may provide information to the controller 120, as indicated by line112, such as set-points and limits of the amount of liquid available,temperature, pressure, concentration, density, etc., and possibly aninitial flow rate of the liquid. The controller 120 may be adapted touse such information, and other inputs, to adjust the flow rate ofliquid in liquid line 104.

In an exemplary embodiment of a flow control system according to thepresent invention, a bubble detection module 124 may analyze one or moresignals provided by the flow meter as represented by line 118. Thebubble detection module 124 may then pass information to the controller120 as illustrated by line 122. The information passed to controller 120may include the current flow meter signal 118, one or more prior flowmeter signals, and/or an alert signal (such as a Boolean flag)indicative of the presence or absence of a bubble in the flow meter 110.Although the bubble detection module 124 is illustrated in FIG. 1 as adistinct module, in other embodiments it may be incorporated into theflow meter 110, or into the controller 120. Where the bubble detectionmodule 124 is incorporated into the flow meter 110 it may analyze theflow meter signal for the presence of a bubble prior to passinginformation to the controller. In embodiments in which the bubbledetection module is incorporated into the controller 120, it may analyzethe incoming signal from the flow meter 118 for the presence of a bubbleso that the controller may respond accordingly as described in moredetail further below.

In addition, as illustrated in FIG. 1, the controller 120 may be coupledto a user interface 128 which may be, for example, a graphically baseduser interface. The user interface 128 may allow a user to monitor thesystem and to provide input to the controller 120, as indicated by line126. The user may be able, via the user interface 128, to observeparameters of the system (provided by the controller 120 or by the flowmeter 110) and/or to provide inputs to the controller 120 such as, forexample, a desired flow rate of the liquid, (i.e., liquid flow rate setpoints) and/or upper and lower flow rate limits. The controller 120 mayoutput to the user interface 128 various information including, forexample, actual flow rate, out-of-limit alarms, and data management anddata decision support information. It should be appreciated that thecontroller 120 may be coupled to another system computer instead of, oras well as, being coupled to the user interface 128. The bubbledetection module 124 may also be coupled to user interface 128, asillustrated schematically by line 130. Thus, for example, the userinterface 128 may respond to receipt from the bubble detection module124 of a positive alert signal indicative of the presence of a bubble bydisplaying an alert. In embodiments in which the bubble detection moduleis incorporated into the flow meter 110 or the controller 120, the userinterface may receive the bubble alert status directly from the flowmeter 110 or the controller 120.

It is to be appreciated that the controller 120 may be programmed withone of a variety of programs for controlling the valve 108. For example,the controller may be programmed to utilize proportional integral (PI)control, proportional integral differential (PID) control, etc., suchas, for example, described in detail in connection with a thermal massflow meter/controller in commonly-owned U.S. Pat. No. 6,962,164, whichis herein incorporated by reference in its entirety. In another example,the controller may be adapted to use a “model-free” adaptive controlalgorithm to drive the valve 108. This method includes a feedback“neuron-based” control algorithm that is independent of the particularliquid flowing in the system and does not require a priori knowledge ofthe dynamics of the system. At least one embodiment of this method isdescribed in detail in U.S. Pat. No. 6,684,112 to George Shu-Xing Cheng,which is herein incorporated by reference in its entirety.

According to one embodiment, the flow meter 110 may be an ultrasonicflow meter that is adapted to use parameters of ultrasonic wavespropagated through the liquid to determine the flow rate of the liquid.Such parameters may include the amplitude, frequency, and/or thetime-of-flight of ultrasonic signals propagating through the liquid.Illustrative examples of the manner in which ultrasonic signals may beused to determine the flow rate of a liquid are described, for example,in commonly owned U.S. application Ser. No. 10/878,974 entitled“Ultrasonic Liquid Flow Controller,” published as US20050288873A1, whichis herein incorporated by reference in its entirety. In otherembodiments, other types of flow meters may be employed, includingwithout limitation coriolis force flow meters, thermal flow meters, orother flow meters known in the art.

FIG. 2 is a flow diagram functionally illustrating, in broad overview,an exemplary method that may be performed by a flow meter and/or flowcontroller system to detect and respond to the presence of a bubble in asensor conduit. The system may begin (block 200) at startup or withinput from a user, for example, initiating flow control. In an exemplaryembodiment, the system periodically measures the flow through the flowmeter, receiving a flow meter signal (block 202). The nature of the flowmeter signal will depend upon the type of flow meter used in the system.The received flow meter signal may be either a raw signal from the flowmeter, a calibrated signal corresponding to the liquid flow rate in thesystem as a whole, or some combination of raw signal parameters andcalibrated measurements. In some embodiments, the received flow metersignal may be recorded or stored in a memory associated with thecontroller 120, the flow meter 110, or the bubble detection module 124.

In block 204, the system analyzes the flow meter signal and determineswhether the signal indicates that a bubble is present in the sensorconduit of the flow meter (block 208). The presence of a bubble caninterfere with the flow meter signal in characteristic ways that make itpossible to analyze the flow meter signal (or the evolution of flowmeter signal parameters over time) to detect a bubble's entry intoand/or exit from the sensor conduit. Examples of such characteristicchanges in the flow meter signal are discussed further below.

Where the analysis of the flow meter signal or of a number of receivedflow meter signals indicates that a bubble is present in the sensorconduit, an exemplary system may respond in a variety of ways, dependingupon whether the flow meter and bubble detection system are deployed aspart of a flow control system such as that illustrated in FIG. 1, or arebeing used to display, record, or log measurements of parameters of theliquid flow through the system (block 210). Where the flow meter isbeing used to display, record, or log measurements of liquid flow, thenupon detection of a bubble, the system may provide a flow rate signalthat is based upon one or more flow rate signals recorded prior to thedetection of the bubble. For example, the system may freeze thedisplayed, recorded, or logged value at the last flow rate measuredprior to detection of a bubble, or at an average of some fixed number offlow rate measurements taken prior to the detection of the bubble (block212). In this way, spurious readings of flow rate that occur as a resultof the bubble's disruption of the operation of the flow meter will notbe displayed or recorded. Instead of or in addition to freezing the flowrate (block 212), the flow meter may also generate an alert signal thatcan be used to notify a user and/or other parts of the system that abubble has been detected (and, therefore, that any flow ratemeasurements provided during the presence of the bubble may not beaccurate). For example, the alert signal may be a Boolean flag that canbe set and passed to the controller (e.g., as a separate signal or as apart of the signal 122 in FIG. 1) and/or to the user interface (i.e.,signal 130 in FIG. 1) and/or written to a measurement log.

Where the flow meter and bubble detection system are incorporated into aflow control system (i.e., Flow Control YES), then the system mayrespond to the detection of a bubble in the sensor conduit by freezingthe flow control parameters by which the controller 120 controls thecontrollable valve 108 (block 214). In this way, instead of the flowcontroller tracking and responding to spurious flow meter signals causedby the presence of a bubble in the sensor conduit, stable flow can bemaintained while the bubble is present. Freezing the control parameters(block 214) can be achieved in a variety of ways. In one exemplaryembodiment, the controller 120 receives from the bubble detection module124 a positive alert signal indicating the presence of a bubble, andfreezes the control parameters (such as the controllable valve setting)in response to that signal. In other embodiments (including embodimentsin which the bubble detection module 124 is integral with the flow meter110), the control parameters may be effectively frozen by providing tothe controller 120 a fixed flow meter signal, for example, based upon aprior flow meter signal. In an embodiment in which the controller 120computes control parameters based upon the flow meter signal itreceives, it will not alter the control parameters as long as it isreceiving a fixed flow meter signal. This fixed flow meter signal may bethe last flow meter signal collected prior to detection of a bubble.Alternatively, the fixed flow meter signal may be a weighted sum (suchas an average) of a number of prior recorded flow meter signals, or, itmay be any fixed flow meter signal that is suitable to stabilize thecontroller response (and hence the flow through conduit 104) while abubble is present in a sensor conduit.

FIG. 3 is a flow diagram illustrating the response of an exemplaryembodiment of a flow meter or flow controller system to the bubble'sexit from the sensor conduit. Having frozen the control parametersand/or the flow measurement upon detection of a bubble in block 214(FIG. 2), an exemplary system may now await detection of flow metersignals characteristic of the bubble's exit from the sensor conduit(block 300). The system may periodically record signals from the flowmeter (block 302) and analyze these signals (block 304) for featurescharacteristic of a bubble's exit from the sensor conduit. As in thecase of detecting the bubble's presence in the sensor conduit, thesystem may analyze the flow meter signal, a collection of a number ofrecorded flow meter signals, and/or the time evolution of flow metersignal parameters in order to detect changes characteristic of abubble's exit from the sensor conduit as discussed in further detailbelow. The system may respond in a number of different ways to detectionof the bubble's exit (step 308), depending upon whether the flow meterand bubble detection system are deployed as part of a flow controlsystem such as that illustrated in FIG. 1, or are being used passivelyto display, record, or log measurements of liquid flow through thesystem (block 310). In an exemplary embodiment in which the flow meterand bubble detection system are being used to display, record, or logmeasurements of liquid flow, upon detection of the bubble's exit thesystem may simply resume the display, recording, or logging of newmeasurement values (block 318). In some embodiments, the flow metersystem may (in addition to or instead of resuming display or recordingof measurements) reset the alarm signal and/or send a signal indicatingthe absence of a bubble. Such a signal may be received by, for example,the user interface 128 and/or the controller 120.

In embodiments in which the flow meter and bubble detection system areincorporated into a flow control system, the system may respond to thedetection of a bubble's exit from the sensor conduit by resuming flowcontrol, i.e. allowing the controller 120 to alter the flow controlparameters in response to flow meter signals (block 320). This can beachieved in a variety of ways. In one exemplary embodiment, thecontroller 120 receives from the bubble detection module 124 a signalindicating the exit of the bubble (e.g., the negation of the alarmsignal generated in response to detection of a bubble), and in responseto that signal resumes dynamic flow control. In other embodiments(including embodiments in which the bubble detection module 124 isintegral with the flow meter 110), the control parameters may beeffectively unfrozen by resuming the provision of current flow metersignals from the flow meter 110 to the controller 120.

In either case—whether the flow meter and bubble detection system aredeployed as part of a flow control system such as that illustrated inFIG. 1, or are being used passively to display, record, or logmeasurements of liquid flow through the system—the system may include adelay between the detection of the bubble's exit and the resumption ofdynamic flow control and/or display or recordation of measurements(blocks 312, 314). Such a delay may be useful to allow any disruption ofthe flow meter signal, flow rate, or of smooth, laminar flow caused bythe bubble's passage through the sensor conduit to subside beforeresuming the recording of measurements or before resuming dynamic flowcontrol.

Thus, in an exemplary embodiment, in response to detection of thebubble's exit from the sensor conduit (block 308), a countdown may bestarted for a predetermined amount of time (or, in the case of a digitalcontrol loop, a predetermined number of process or thread cycles), andthe suspended flow meter measurements and/or controller parameters maybe extended until the countdown is over. This delayed latching allowsany spurious signals to subside before resuming measurement and/or flowcontrol after the bubble clears the sensor conduit. In an exemplaryembodiment the delay is optimized so as not to be so small that thespurious signals or flow turbulence may not yet have subsided, but alsonot to be so long that the flow parameters have an opportunity to driftaway from the setpoint. In an exemplary embodiment, a delay of about100-150 ms is sufficient to allow any spurious signals or flowturbulence to dissipate without unnecessarily delaying the resumption ofreal-time measurement and control, but longer delays may be necessaryfor some embodiments. The appropriate delay will depend upon theparticular implementation of the flow meter and flow control system,including such parameters as flow rate, liquid viscosity, typical bubblesize, the diameter or orientation of the sensor conduit, and others.

An exemplary flow meter or flow control system may also be adapted torespond to a second bubble entering the sensor conduit during thecountdown process initiated by the exit of the first bubble. In oneembodiment, where a second bubble enters the sensor conduit shortlyafter the exit of a first bubble, i.e., during the delay periodrepresented by blocks 312 and/or 314, the resumption of flow metermeasurements and/or controller parameters may be delayed further. Oncethe second bubble's exit is detected a new countdown may begin.

In alternative embodiments, instead of detecting the bubble's exit fromthe sensor conduit as described above, the flow meter and/or flowcontroller system may be configured to reset the alarm signal, resumemeasurement logging, and/or resume dynamic flow control after a presetamount of time or a preset number of process or thread cycles. Anappropriate preset duration can be determined empirically, and maydepend upon various system parameters, including such parameters as flowrate, liquid viscosity, typical bubble size, the diameter or orientationof the sensor conduit, and others.

In certain situations, for example where the flow is characterized bylow flow rates and/or for low liquid pressures, a bubble may take arelatively long time to transit through the sensor conduit, causing aprolonged disruption of accurate flow parameter sensing. For thatreason, some embodiments of flow controllers may include means for thecontrolled forcing of a bubble from the sensor conduit, particularly ininstances where it appears that the bubble has become stuck. Controlledforcing typically involves temporarily altering the flow rate throughthe sensor conduit to urge the bubble out of the conduit. In anexemplary embodiment, the means for controlled forcing can includeprogram code to rapidly open and close the valve in order to knock thebubble from the conduit.

In some embodiments, the controller may begin counting time and/orthread cycles upon detection of the presence of a bubble in the flowcontrol system or upon receipt from the flow meter or the bubbledetection module of an alarm signal indicating the presence of thebubble. In addition, the controller may be programmed or provided (e.g.,through the user interface or other data source) with an expectedtransit time of a bubble through the sensor conduit. With thatinformation, the system can respond with a controlled forcing procedurewhere the bubble's passage takes much longer than the expected transittime. The estimated transit time of a bubble can depend upon a varietyof system parameters (such as liquid flow rate, viscosity, conduitdiameter, etc.) and also upon the size of the bubbles that tend to occurin the system. For example, smaller bubbles move more slowly, as liquidtends to move around them rather than push them along the conduit. Incontrast, larger bubbles tend to flow along with the liquid flow rate,but (depending upon the ratio of bubble size to conduit diameter) can beprone to becoming stuck in the conduit and/or causing unstable,turbulent, or slug flow. It should be appreciated that dependent on theorientation of the sensor conduit (e.g., whether the sensor conduit isdisposed in a direction aligned with or transverse to a force of gravityor other force of acceleration) certain implementations may include aprovision to detect and/or respond to a stuck bubble, while others maynot.

In one exemplary embodiment, where a time larger than the expectedtransit time has passed without the system detecting the bubble's exit,the controller may, in response, execute a rapid opening and closing ofthe control valve, momentarily increasing the flow rate through thesensor conduit to force the bubble out. In such an embodiment,closed-loop operation may be suspended prior to executing this procedureand resumed thereafter, returning flow rate to its controlled level.Where, after execution of a controlled forcing procedure, analysis ofthe flow meter signal indicates that the bubble is still within the flowconduit, the controlled forcing procedure may be repeated until acharacteristically normal flow signal is achieved and/or thebubble-detection signals indicate that a bubble is no longer present.

Unlike a flow controller, a passive flow meter is not capable of forcinga blocked bubble out of a sensor conduit. In flow meter applications, aconfigurable alarm output may be used to signal an abnormal or disruptedflow condition where a bubble appears to have become stuck in the sensorconduit. In some embodiments, such a flag may be set and passed to auser interface, and/or recorded or logged along with measurements fromthe flow meter.

As described above, a bubble's presence in or exit from a sensor conduitcan be detected by analysis of flow meter signal parameters forcharacteristic changes. An exemplary bubble detection system is nowdescribed.

One example of a flow controller that is known in the art employs anultrasonic flow sensor to provide liquid flow rate feedback to thecontroller. Ultrasonic flow sensors measure liquid flow rate through asensor conduit by propagating one or more ultrasonic signals through theliquid and measuring one or more effects that the flowing liquid canhave on the propagating signals. For example, ultrasonic liquid flowsensors may detect changes in frequency, phase, or time of ultrasonicsignals propagating through a flowing liquid to determine a transit time(time-of-flight) to measure flow rate. Such ultrasonic sensors andcontrollers incorporating them are described, for example, in U.S. Pat.Nos. 6,055,868, 5,974,897, and 3,575,050, as well as publishedapplication US2005/0288873A1.

An example of characteristic signals indicating the presence of a bubblein an exemplary ultrasonic flow sensor is illustrated in FIGS. 7A-7G.The ultrasonic flow sensor 700 includes a sensor conduit 712 andultrasonic transducers 702 and 704. An alternative sensor conduit 1000for use in an ultrasonic flow meter may have a different shape, such asthat depicted in FIG. 10 and described in the commonly-owned, copendingU.S. patent application entitled “Ultrasonic Flow Sensor” by Thomas OwenMaginnis and Kim Ngoc Vu, filed under Attorney Docket NumberC1138-700910 on Aug. 10, 2006, and incorporated by reference herein. Inthe illustrated embodiment of FIG. 7A, liquid flows through the conduit712 along flow direction 710. During operation, ultrasonic signals aregenerated at each of the transducers 702 and 704 and permitted topropagate through the flowing liquid for detection by the othertransducer. The liquid flow rate can be determined from, for example,cross-correlation analysis of the frequency or phase, or the timedifference of ultrasonic signals propagating along the flow direction710 compared to those propagating against the flow direction 710 toestablish a time-of-flight.

In an exemplary embodiment, a chirp signal such as that represented byreference number 720 in FIG. 7A is generated at transducer 702 andpermitted to propagate through the liquid to transducer 704. Changes inits amplitude may be analyzed to determine whether a bubble 708 ispresent in the sensor conduit 712. These changes are shown schematicallyin FIGS. 7A-7G and are plotted schematically against time in FIG. 8.(Although this discussion considers the exemplary case in which theultrasonic signal propagates along the flow direction, in someembodiments the chirp signal 720 is generated at transducer 704 andpropagates upstream to transducer 702. In further embodiments, signalspropagating in both directions are employed.)

In FIG. 7A, before a bubble 708 enters the sensor conduit, the receivedsignal amplitude is relatively steady with time (point A in FIG. 8). Asthe bubble 708 approaches the entrance to the measurement section of thesensor conduit, as shown in FIG. 7B, the amplitude of the receivedsignal momentarily increases, as illustrated by reference number 722 inFIG. 7B and at point B in FIG. 8. This momentary increase in theamplitude of the received signal does not appear to be related to achange in the gain applied to received flow signal by the flow meterelectronics (because it occurs over a period of time which is typicallymuch smaller than the response time of the gain control in the flowmeter electronics), and is believed to be attributable to the ultrasonicsignals reflecting back from the bubble 708 at the entrance to theconduit 712 and reinforcing in phase with the transmitting wavefrontthat is traveling towards the direction of the transducer 704.Regardless of the physical mechanism, the observed change in signalamplitude is characteristic of the presence of a bubble, and thus can beused to detect the bubble.

As the bubble 708 reaches the transducer 702, as shown in FIG. 7C, theamplitude of the signal detected at transducer 704 begins to attenuatesharply, as illustrated by reference number 724 (point C in FIG. 8).FIG. 7D illustrates the received signal 728 when the bubble 708 istraversing the central region of the sensor conduit 712 (point D in FIG.8). These figures illustrate why a system for compensating for thepresence of the bubble 708 is of value; in the absence of such a system,these transient changes in signal amplitude will appear to the controlsystem as changes in flow rate for which the control system will attemptto compensate, which may result in unstable flow.

As the bubble 708 reaches and passes the transducer 704, as shown inFIG. 7E, the received signal 730 rises sharply (point E in FIG. 8).Generally, the signal level may overshoot before stabilizing to a steadystate value. The overshoot may be attributed at least in part to thereinforcement in phases of the primary ultrasound wavefront travelingfrom the transducer 702 and the reflected wave from the bubble 708 as itreaches the transducer 704. The reflected signal fades away as thebubble 708 moves through the measurement section of the sensor conduitas shown in FIG. 7F (point F in FIG. 8). Finally, as shown in FIG. 7G,as the bubble 708 exits the sensor conduit 712, the received signalamplitude 734 returns to its steady state value (point G at FIG. 8).

An exemplary bubble detection process takes advantage of thecharacteristic time evolution of the detected signal amplitude, such asthat illustrated in FIGS. 7A-7G and FIG. 8, to detect the presence ofthe bubble 708. It should be appreciated that the time evolution of thedetected signal amplitude illustrated in FIGS. 7A-7G and FIG. 8 reflectsmeasurements obtained with a particular flow sensor over a particularrange of flow rates, and that the characteristic time evolution of thedetected signal amplitude may vary when used with other sensorconfigurations and other flow rates. Empirical measurements may be madewith other sensor configurations and at other flow rates to identify themanner in which the presence of a bubble can affect the detected signalamplitude, and a bubble detection process may then be adapted to reflectthat characteristic signature.

In accordance with the present invention, an exemplary bubble detectionprocess, illustrated functionally in FIG. 4, is provided that comparessuccessive detected signal amplitudes to previous signal amplitudes,looking for the characteristics indicative of the presence of a bubble,such as those illustrated in FIGS. 7A-7G and FIG. 8. In one exemplaryembodiment, a bubble detection system may receive signal amplitudevalues from the flow meter (block 402). In one embodiment, a number ofreceived signal amplitudes may be stored in a memory associated with theflow meter, the flow controller, or the bubble detection module. Thesystem periodically compares the received amplitude to a previouslyreceived and stored amplitude or, alternatively, to multiple previouslyreceived and stored amplitudes (block 404). Where the system detects acharacteristic change in signal amplitude (such as that illustrated inFIGS. 7A-7G and FIG. 8) (block 408), it may, in response, generate analarm signal (block 410) as described above.

A further exemplary embodiment computes a weighted sum of the detectedflow meter signal amplitude and, where the received value of theamplitude of the flow meter signal amplitude deviates from the weightedsum, for example, by some predetermined threshold or amount, sends inresponse a positive bubble detection signal. For example, the embodimentillustrated in FIG. 5 computes a running average of the detectedultrasonic signal amplitude and, where the received value of theamplitude of the flow meter signal deviates from the running average,sends in response a positive bubble detection signal. It should beappreciated that although a running average of the detected ultrasonicsignal amplitude is used in this embodiment, other forms of weightedsums of the detected ultrasonic signal amplitude may be used, as thepresent invention is not so limited. For example, rather than simplycomputing a running average, more recently received values of theamplitude of the flow meter signal may be accorded a different (e.g.,higher) weight than those that were received less recently. The weightedsum may be computed by a processor associated with the flow meter, theflow controller, or the bubble detection module, although it should beappreciated that such a weighted sum could alternatively be determinedby filtering the received flow meter signals using, for example, a lowpass filter.

In accordance with the embodiment depicted in FIG. 5, the amplitude ofthe received signal may be periodically sampled and stored in a buffer(block 502), sampling the flow measurement at some predetermined timeinterval. Some number N of samples (e.g. 20 samples in one exemplaryembodiment) may be stored. In one exemplary embodiment, the signalsamples may be stored in a stack such that as the new received signal issampled, the new data is placed on top and the old data is pushed downthe stack. After the data is collected for the predetermined number of Nsamples, the average signal amplitude is calculated. An advantage ofdetermining a running average amplitude in real time, rather thanstoring a preset value, is that different sensor configurations may leadto different signal strengths, depending upon the physical mountings ofthe sensors, the flow parameters in the particular arrangement, andother factors. Thus, while a predetermined value of the amplitude (i.e.,determined during calibration, based upon measurements from similarsensors, etc.) may be employed as a reference, determining a runningaverage signal amplitude in real-time is preferred.

As new samples are taken, the newest sample is stored at the top of thestack, and the oldest sample (at the bottom of the stack) is discarded.The running average of N samples in the buffer is calculatedperiodically (block 504). In an exemplary embodiment the running averagesignal amplitude may be computed each time a new sample is placed in thebuffer.

In one exemplary embodiment, a threshold level window that extends aboveand below the running average value is selected as a bubble detectionthreshold, such that upon the received value of the amplitude of theflow meter signal exceeding this threshold or dipping below it, a bubbleis presumed to have been detected and a positive bubble detection signalis generated (blocks 508, 510). The threshold may be selected to be somepercentage deviation above and below the running average value. Forexample, the threshold may be a 10, 15, or 20 percent deviation from therunning average value. The threshold value can be larger or smallerdepending upon a number of system parameters and operating conditions.The threshold need not be symmetric about the running average value.Factors that contribute to the determination of the threshold value mayinclude (without limitation) viscosity of the liquid whose flow is beingmeasured, operating temperatures and pressure, particle contents whenthe liquid is a slurry, etc. In practice, an optimal threshold can bedetermined by trial and error or during calibration, for example, bydeliberately injecting bubbles into the sensor conduit and setting athreshold level that allows the desired efficiency of bubble detection(i.e., minimizing the number of undetected bubbles and the number offalse positives). The comparison of the received value of the amplitudeof the flow meter signal to a threshold reduces the possibility of afalse positive bubble detection.

In accordance with the embodiment depicted in FIG. 5, where the receivedamplitude value of the flow signal exceeds the upper threshold value orfalls below the lower threshold value, the system in response generatesa positive bubble-detection signal (block 508). In one exemplaryembodiment, a positive bubble-detection signal may simply entailasserting a bubble-exist flag (i.e. setting a Boolean bubble-existvariable to some particular value). The bubble-exist flag may remainasserted until the bubble leaves the sensor conduit (or at least themeasurement section of the sensor conduit). The bubble-exist flag can beused as a trigger signal for other control and data storage operations,such as freezing control parameters or freezing recorded measurementvalues, as described above.

In an exemplary embodiment, upon assertion of the bubble-exist flag, thesystem may, in response, start a timer that counts time (CPU clockcycles, or a number of process or thread cycles). This timer can be usedto determine whether assertion of the bubble-exist flag is due to a truebubble being present, or to a momentary glitch in the data collection.That is, where the bubble-exist flag remains asserted for a time that isless than a predetermined minimum number of clock cycles or threadcycles, the system may, in response, treat the excursion beyondthreshold as a glitch rather than a true bubble event. On the otherhand, where the bubble-exist flag is asserted for a time longer thansome predetermined maximum interval (or number of clock or threadcycles), it is likely that the bubble has blocked the sensor conduit andstopped the flow entirely. In such a circumstance, the system may, inresponse, initiate an action for the controlled forcing of the bubble toreturn the flow to the normal steady-state operating condition, asdiscussed above. The predetermined maximum interval (or number ofcycles) at which a controlled forcing procedure may be initiated dependsupon operating conditions such as flow rate, liquid viscosity, etc.

In some embodiments, it is desirable to detect the presence of a bubblevery early in its transit through the sensor conduit. In particular,where the ultrasonic flow meter is used as the sensor component in aflow controller, it is desirable to detect the bubble as soon aspossible before it disrupts the control signal, preferably before thecontroller has had a chance to alter the feedback parameters (and hencethe flow rate) in response to the spurious signals caused by thepresence of the bubble.

In some cases, the moving or running average threshold detectiondescribed above will sense the bubble in a timely manner, and thecontroller will respond to the positive bubble-detection signal, holdingthe controller parameters until the bubble completes its transit throughthe sensor conduit. However, as illustrated in FIG. 7 b and at point Bin FIG. 8, where the amplitude of the signal rises sharply, it can bebeneficial to alert the controller about this signal spike early, beforethe running average signal amplitude (or some other weighted sum)reflects it, and before the control system reacts to the changes. Oneexemplary embodiment of a flow meter with bubble detection may thereforeemploy a differential threshold detection scheme that assists indetecting bubbles at very early stages, before the controller hascommitted to the change of operating parameters.

In one embodiment, the differential threshold detection is based on thechange in the value of the time-of-flight delay difference parameter,denoted herein as x₀, which is more sensitive than the amplitude change.The time-of-flight delay difference parameter x₀ is the differencebetween the transit time of an ultrasonic signal (for example, a chirpsignal) traveling from transducer 702 to transducer 704 and the transittime of an ultrasonic signal traveling in the reverse direction fromtransducer 704 to transducer 702 (that is, the difference between thedownstream transit time and the upstream transit time). The differencebetween the current value x₀[t] and a previous value x₀[t−1] may beperiodically computed and this resultant difference then compared to apredetermined threshold (blocks 608, 610).

FIG. 9 illustrates the corresponding plot of the time-of-flightdifference values x₀[t]−x₀[t−1] against time for the entire bubbletransit sequence illustrated in FIGS. 7A-7G and 8. It can be seen fromFIG. 9 that when using the x₀ differences, the presence of a bubbleappears as a sharp spike very close to the beginning of thecharacteristic signal sequence observed in FIG. 8.

An exemplary embodiment of a bubble detection process employing such adifferential measurement is illustrated functionally in FIG. 6. Valuesof the time-of-flight delay difference parameter x₀ may be computedperiodically by measuring the downstream time-of-flight (block 602),measuring the upstream time-of-flight (block 604), and taking thedifference (block 606). It should be appreciated that blocks 602 and 604may be performed in the reverse order or simultaneously.

In one exemplary embodiment the time-of-flight difference x₀[t]−x₀[t−1]is computed each time a new x₀ value is obtained. Upon a determinationthat the difference exceeds a predetermined threshold (the differentialthreshold), then a bubble is suspected and a positive bubble detectionsignal, such as asserting a bubble-exist flag, can be generated (blocks610, 612). Alternatively, to minimize the incidence of false positiveresponses, the differential threshold condition may be checked again atthe next thread cycle, and a positive bubble detection signal generatedonly where the threshold condition is again met. In either case, thethreshold value may be selected to be greater than the maximum change inx₀ that could occur due to the step changes in system parameters such asset point values, so that false positive bubble-detection signals arenot generated when the flow parameters are deliberately changed.Alternatively, differential bubble detection may be temporarily lockedout or suspended during changes in setpoint values.

A positive bubble detection signal generated as a result of a change inx₀ that exceeds the differential threshold may be used as describedabove to freeze feedback parameters or measurement values, preventingdestabilizing system response to spurious signals generated by thepresence of the bubble.

The differential detection criterion may detect a bubble's presenceearlier than running average detection, and therefore may provide anearly warning, allowing the system to freeze the controller statesbefore the controller states are corrupted by the spurious signalscaused by the presence of the bubble. Even where the differentialdetection precedes running average detection by as little as one or twothread cycles, it can still be useful in facilitating stable operationof a control system during transit of a bubble.

In an exemplary embodiment, the differential detection supplements therunning average detection described previously above, and is mainlyactive during the bubble's entry into the sensor conduit. However, itcan also be used to detect the bubble's exit from the sensor conduit, asillustrated by the second, smaller peak in FIG. 9.

Although the present invention has been primarily described herein asbeing used to provide accurate measurement and/or control of fluid flowrate in the presence of bubbles, aspects of the present invention may beused for other purposes where the presence of bubble may affect ameasurement and/or control process. For example, U.S. Pat. No. 5,569,844describes the use of ultrasonic waves to measure the particle size anddistribution of solids in a suspension of solids in solution, such as aCMP slurry. The '844 patent describes that the presence of bubbles canlimit the accuracy of such measurements, and notes that bubblestypically need to be eliminated prior to measurement. Aspects of thepresent invention may be readily adapted to such a particle size anddistribution measurement scheme to detect the presence of bubbles,and/or to limit particle size and distribution measurements to periodsin which bubbles are not detected. Adaptations to other technologieswhere the presence of bubbles may impact measurement and/or control mayalso be readily envisioned.

Having thus described several aspects of embodiments of the presentsystems and methods for measuring and controlling liquid flow, it is tobe appreciated various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only.

1. A method of measuring a flow rate of a liquid flowing in a liquidflow sensor, the liquid flow sensor including a sensor conduit and theliquid including a plurality of bubbles formed therein, the methodcomprising acts of: repeatedly sensing a flow signal indicative of theflow rate of the liquid flowing in the sensor conduit; determining,based upon at least one parameter of the flow signal, whether at leastone bubble is disposed in the liquid within the sensor conduit;providing, in response to a determination that no bubble is disposed inthe liquid within the sensor conduit, a flow rate signal indicative ofthe flow rate of the liquid flowing in the liquid flow sensor based upona most recently sensed flow signal; and providing, in response to adetermination that the at least one bubble is disposed in the liquidwithin the sensor conduit, at least one of: a) a flow rate signal basedupon the most recently sensed flow signal and an alert signal indicativeof a presence of the at least one bubble, and b) a flow rate signalindicative of the flow rate of the liquid flowing in the liquid flowsensor based upon other than the most recently sensed flow signal. 2.The method of claim 1, wherein the act of determining includes an act ofdetecting at least one change in the at least one parameter of the flowsignal, the change being characteristic of the presence of the at leastone bubble disposed in the liquid within the sensor conduit.
 3. Themethod of claim 1, wherein the liquid flow sensor comprises anultrasonic liquid flow sensor.
 4. The method of claim 3, wherein the atleast one parameter of the flow signal comprises an ultrasonic flowsignal amplitude.
 5. The method of claim 4, wherein the act ofdetermining includes an act of detecting a change in the ultrasonic flowsignal amplitude, the change being characteristic of the presence of theat least one bubble disposed in the liquid within the sensor conduit. 6.The method of claim 5, further comprising an act of: calculating,responsive to the act of repeatedly sensing, a running average of avalue of the ultrasonic flow signal amplitude; wherein the act ofdetermining includes an act of determining whether a most recentlysensed value of the ultrasonic flow signal amplitude deviates from therunning average by more than a threshold value.
 7. The method of claim3, wherein the flow signal comprises an ultrasonic time-of-flightdifference flow signal, and wherein the at least one parameter of theflow signal includes a magnitude of the ultrasonic time-of-flightdifference flow signal.
 8. The method of claim 7, wherein the act ofdetermining includes an act of determining whether the magnitude of theultrasonic time-of-flight difference flow signal deviates from themagnitude of a prior ultrasonic time-of-flight difference flow signal bymore than a threshold value.
 9. The method of claim 1, furthercomprising an act of: calculating, responsive to the act of repeatedlysensing, a weighted sum of a value of the at least one parameter of theflow signal; wherein the act of determining includes an act ofdetermining whether a most recently sensed value of the at least oneparameter of the flow signal deviates from the weighted sum by more thana determined amount.
 10. The method of claim 1, further comprising anact of: calculating, responsive to the act of repeatedly sensing, arunning average of a value of the at least one parameter of the flowsignal; wherein the act of determining includes an act of determiningwhether a most recently sensed value of the at least one parameter ofthe flow signal deviates from the running average by more than athreshold value.
 11. The method of claim 1, wherein the act of providingthe flow rate signal indicative of the flow rate of the liquid flowingin the liquid flow sensor based upon other than the most recently sensedflow signal includes providing a previously sensed flow signal for whichit was previously determined that no bubble was disposed in the liquidwithin the sensor conduit.
 12. The method of claim 11, wherein thepreviously sensed flow signal comprises the most recently sensed flowsignal for which it was previously determined that no bubble wasdisposed in the liquid within the sensor conduit.
 13. The method ofclaim 1, wherein the act of providing the flow rate signal indicative ofthe flow rate of the liquid flowing in the liquid flow sensor based uponother than the most recently sensed flow signal includes providing apreviously sensed flow signal for which it was previously determinedthat no bubble was disposed in the liquid within the sensor conduitalong with the alert signal.
 14. A method of controlling the flow rateof a liquid through a flow conduit coupled to a controllable valve and aliquid flow sensor, liquid flow sensor including a sensor conduit andthe liquid including a plurality of bubbles formed therein, the methodcomprising acts of: repeatedly sensing a flow signal indicative of theflow rate of the liquid flowing in the sensor conduit; determining,based upon at least one parameter of the flow signal, whether at leastone bubble is disposed in the liquid within the sensor conduit;providing, in response to a determination that no bubble is disposed inthe liquid within the sensor conduit, control parameters for thecontrollable valve based upon the most recently sensed flow signal;providing, in response to a determination that the at least one bubbleis disposed in the liquid within the sensor conduit, control parametersfor the controllable valve based upon other than the most recentlysensed flow signal; and controlling the controllable valve according tothe control parameters.
 15. The method of claim 14, wherein the act ofdetermining includes detecting at least one change in the at least oneparameter of the flow signal, the change being characteristic of apresence of the at least one bubble disposed in the liquid within thesensor conduit.
 16. The method of claim 14, wherein the liquid flowsensor comprises an ultrasonic liquid flow sensor.
 17. The method ofclaim 16, wherein the at least one parameter of the flow signalcomprises an ultrasonic flow signal amplitude.
 18. The method of claim17, wherein the act of determining includes detecting a change in theultrasonic flow signal amplitude, the change being characteristic of apresence of the at least one bubble disposed in the liquid within thesensor conduit.
 19. The method of claim 18, further comprising an actof: calculating, responsive to the act of repeatedly sensing, a runningaverage of a value of the ultrasonic flow signal amplitude; wherein theact of determining includes an act of determining whether a mostrecently sensed value of the ultrasonic flow signal amplitude deviatesfrom the running average by more than a threshold value.
 20. The methodof claim 16, wherein the flow signal comprises an ultrasonictime-of-flight difference flow signal, and wherein the at least oneparameter of the flow signal includes a magnitude of the ultrasonictime-of-flight difference flow signal.
 21. The method of claim 20,wherein the act of determining includes an act of determining whetherthe magnitude of the ultrasonic time-of-flight difference flow signaldeviates from the magnitude of a prior ultrasonic time-of-flightdifference flow signal by more than a threshold value.
 22. The method ofclaim 14, further comprising an act of: calculating, responsive to theact of repeatedly sensing, a weighted sum of a value of the at least oneparameter of the flow signal; wherein the act of determining includes anact of determining whether a most recently sensed value of the at leastone parameter of the flow signal deviates from the weighted sum by morethan a determined amount.
 23. The method of claim 14, further comprisingan act of: calculating, responsive to the act of repeatedly sensing, arunning average of a value of the at least one parameter of the flowsignal; wherein the act of determining includes an act of determiningwhether a most recently sensed value of the at least one parameter ofthe flow signal deviates from the running average by more than athreshold value.
 24. The method of claim 14, wherein the act ofproviding control parameters based upon other than the most recentlysensed flow signal includes providing previously provided controlparameters for which it was determined that no bubble was disposed inthe liquid within the sensor conduit.
 25. The method of claim 24,wherein the previously provided control parameters comprise the mostrecently provided control parameters for which it was previouslydetermined that no bubble was disposed in the liquid within the sensorconduit.
 26. The method of claim 14, further comprising acts of: waitinga predetermined period of time in response to a determination that theat least one bubble is disposed in the liquid within the sensor conduit;determining, after the predetermined period of time, whether the atleast one bubble is still disposed in the liquid within the sensorconduit, based upon the at least one parameter of the flow signal; andexecuting, in response to a determination that the at least one bubbleis still disposed in the liquid within the sensor conduit, a controlledforce procedure to remove the at least one bubble from the liquid withinthe sensor conduit.
 27. The method of claim 26, wherein the act ofdetermining whether the at least one bubble is still disposed in theliquid within the sensor conduit includes detecting at least one changein the at least one parameter of the flow signal, the change beingcharacteristic of one of: a) a presence of the at least one bubbledisposed in the liquid within the sensor conduit; and b) an exit of theat least one bubble from the liquid within the sensor conduit.
 28. Themethod of claim 26, wherein the act of executing a controlled forceprocedure includes temporarily altering the control parameters for thecontrollable valve.
 29. The method of claim 26, wherein the act ofexecuting a controlled force procedure includes opening and shutting thecontrollable valve.
 30. A system for measuring a flow rate of a liquidflowing in a flow conduit, the liquid including a plurality of bubblesformed therein, the system comprising: a liquid flow sensor including asensor conduit fluidly coupled to the flow conduit, the liquid flowsensor being configured to sense a flow rate of the liquid flowing inthe sensor conduit and provide a flow signal indicative of the flow rateof the liquid flowing in the sensor conduit; a bubble detection module,coupled to the liquid flow sensor, to receive the flow signal anddetermine, based upon at least one parameter of the flow signal, whetherat least one bubble is disposed in the liquid within the sensor conduit,the bubble detection module being configured to: provide, in response toa determination that no bubble is disposed in the liquid within thesensor conduit, a flow rate signal indicative of the flow rate of theliquid flowing in the liquid flow sensor based upon a most recentlysensed flow signal, and provide, in response to a determination that theat least one bubble is disposed in the liquid within the sensor conduit,at least one of: a) a flow rate signal based upon the most recentlysensed flow signal and an alert signal indicative of a presence of theat least one bubble, and b) a flow rate signal indicative of the flowrate of the liquid flowing in the liquid flow sensor based upon otherthan the most recently sensed flow signal.
 31. The system of claim 30,wherein the bubble detection module detects at least one change in theat least one parameter of the flow signal, the change beingcharacteristic of the presence of the at least one bubble disposed inthe liquid within the sensor conduit.
 32. The system of claim 30,wherein the liquid flow sensor comprises an ultrasonic liquid flowsensor.
 33. The system of claim 32, wherein the at least one parameterof the flow signal comprises an ultrasonic flow signal amplitude. 34.The system of claim 33, wherein the bubble detection module detects achange in the ultrasonic flow signal amplitude, the change beingcharacteristic of the presence of the at least one bubble disposed inthe liquid within the sensor conduit.
 35. The system of claim 34,wherein the bubble detection module is further configured to: calculatea running average of a value of the ultrasonic flow signal amplitude;and determine whether a most recently sensed value of the ultrasonicflow signal amplitude deviates from the running average by more than athreshold value.
 36. The system of claim 32, wherein the flow signalcomprises an ultrasonic time-of-flight difference flow signal, andwherein the at least one parameter of the flow signal includes amagnitude of the ultrasonic time-of-flight difference flow signal. 37.The system of claim 36, wherein the bubble detection module is furtherconfigured to: determine whether the ultrasonic time-of-flightdifference flow signal deviates from a previously determined ultrasonictime-of-flight difference flow signal by more than a detection thresholdvalue.
 38. The system of claim 30, wherein the bubble detection moduleis further configured to: calculate a weighted sum of a value of the atleast one parameter of the flow signal; and determine whether the mostrecently sensed value of the at least one parameter of the flow signaldeviates from the weighted sum by more than a determined amount.
 39. Thesystem of claim 30, wherein the bubble detection module is furtherconfigured to: calculate a running average of a value of the at leastone parameter of the flow signal; and determine whether the mostrecently sensed value of the at least one parameter of the flow signaldeviates from the running average by more than a threshold value. 40.The system of claim 30, wherein the flow rate signal indicative of theflow rate of the liquid flowing in the liquid flow sensor based uponother than the most recently sensed flow signal comprises a previouslysensed flow rate signal for which it was previously determined that nobubble was disposed in the liquid within the sensor conduit.
 41. Thesystem of claim 40, wherein the previously sensed flow rate signalcomprises the most recently sensed flow rate signal for which it waspreviously determined that no bubble was disposed in the liquid withinthe sensor conduit.
 42. The system of claim 30, further comprising: acontrollable valve in fluid communication with the flow conduit, tocontrol the flow rate of the fluid flowing in the flow conduit basedupon control parameters provided to the controllable valve; and acontroller, coupled to the liquid flow sensor and the controllablevalve, to receive the flow signal from the liquid flow sensor andprovide the control parameters to the controllable valve.
 43. The systemof claim 42, wherein: the bubble detection module is implemented in thecontroller; wherein, upon the determination that no bubble is disposedin the liquid within the sensor conduit, the controller provides thecontrol parameters to the controllable valve based upon the mostrecently sensed flow signal; and wherein, upon a determination that theat least one bubble is disposed in the liquid within the sensor conduit,the controller provides the control parameters to the controllable valvebased upon other than the most recently sensed flow signal.
 44. Thesystem of claim 30, wherein the bubble detection module provides theflow rate signal based upon the most recently sensed flow signal and thealert signal indicative of the presence of the at least one bubble inresponse to the determination that the at least one bubble is disposedin the liquid with the sensor conduit, the system further comprising: acontrollable valve in fluid communication with the flow conduit, tocontrol the flow rate of the fluid flowing in the flow conduit basedupon control parameters provided to the controllable valve; and acontroller, coupled to the bubble detection module, to receive the flowrate signal and the alert signal and provide the control parameters tothe controllable valve.
 45. The system of claim 44, wherein: in responseto the alert signal, the controller freezes the control parametersprovided to the controllable valve at a prior value.
 46. The system ofclaim 44, wherein: in response to the alert signal, the controller isconfigured to wait for a predetermined period of time, and upon adetermination that the at least one bubble is still disposed in theliquid within the sensor conduit and the predetermined period of timehas elapsed, implement a controlled force procedure to remove the atleast one bubble from the liquid within the sensor conduit.
 47. Thesystem of claim 46, wherein the controlled force procedure comprisesopening and shutting the controllable valve.